Cold cathode fluorescent lights (CCFLs) have been increasingly used as a back light source of a liquid crystal display (LCD) for use in, for example, a monitor of a notebook PC, LCD TV set, and car navigation system. Such a CCFL has substantially the same high efficiency and a long life as a usual hot cathode fluorescent light, without using a filament of a hot cathode fluorescent light.
In order to start up and operate the CCFL, high AC voltages are required. For example, a startup voltage of about 1000 V and an operating voltage of about 600 V (in rms value) are required. (Voltages will be hereinafter given in rms value.) These high AC voltages are generated from a DC power supply provided in, for example, a notebook PC and a liquid crystal TV set, using an inverter.
There has been proposed in Japanese Patent Application Laid Open 2002-233158 (which will be referred to as Patent Document 1) an inverter for providing AC power to a CCFL in the form of a full-bridge type (or H-bridge type) inverter that uses four semiconductor switches. This inverter includes a transformer having a primary winding connected in series with the output end of the H-bridge via a serial resonant capacitor, and a secondary winding to be connected to a load. Of the four semiconductor switches constituting the H-bridge, a first set of two semiconductor switches establishes a current path through the primary winding in a first direction, while a second set of two semiconductor switches establishes a current path through the primary winding in a second direction.
By feeding back the load current flowing through the secondary winding to a control circuit and comparing it with a reference voltage, the inverter generates a control signal consisting of pulses having a fixed pulse width and having controlled relative pulse positions (i.e. have a controlled duty ratio). The control signal is supplied to the semiconductor switching circuit of the H-bridge to regulate the power supplied to the load. The secondary voltage across the secondary winding is monitored to detect an over-voltage that exceeds a predetermined level, and, if it is detected, operation of the inverter is stopped to prevent an over-voltage from occurring. Over-voltage that could occur in the event that the load current lowers below a predetermined level is also prevented by setting the duty ratio to a minimum value.
This type of inverters mostly utilize a battery as a power supply. To charge the battery, a power supply adapter is commonly used. When this power supply adapter is connected to, or disconnected from, the battery, the power supply voltage supplied to the inverter can sharply change (rise or drop). Such sharp change in the power supply voltage can also take place when the magnitude of another load connected to the battery, such as an in-vehicle brake circuit, sharply changes.
In the inverter of the Patent Document 1, when the power supply voltage rises sharply, its display momentarily brightens because it takes some time before the current feedback control takes effect, which gives the viewer of the display discomfort. Moreover, since the load current increases rapidly in the meanwhile, it can cause an undesirable damage to the loads including the CCFL.
Further, when the power supply voltage drops sharply, the load current also decreases accordingly. In the inverter of the Patent Document 1, in order to prevent an over-voltage from being generated due to turning off (or disconnection) of the CCFL, the duty ratio of the semiconductor switching circuit is set to a minimum level if the load current decreases below a predetermined level. In cases where electronic apparatus utilizing this inverter is used in a very cold region for example, the load current of the CCFL can drop and never recover to the normal level, resulting in a shut down of the inverter.
It is therefore an object of the present invention to provide an inverter capable of performing constant-current control on a load connected to a secondary winding of a transformer through pulse width modulation (PWM) of the switches of a semiconductor switching circuit connected to the primary winding of the transformer, thereby suppressing view's discomfort and possible over-current caused by a sharp change (rise and fall) in power supply voltage, or preventing shut down of the inverter. It is another object of the invention to provide a controller IC for use with such inverter. It is a further object of the invention to provide an electronic apparatus equipped with a light emitting apparatus driven by the inverter.